Japanese Patent Document JP-A-4-141988 describes a lighting circuit of a discharge lamp for a vehicle. The lighting circuit uses a DC booster circuit to raise a voltage applied from a battery. A boosting output of the DC booster circuit is connected to a high frequency booster circuit. The high frequency booster circuit is a self-excitation type inverter circuit, and an operating frequency thereof is not changed depending on a control signal. The self-excitation type inverter circuit includes a pair of field effect transistors and a transformer. The boosting output of the DC booster circuit is connected to a center tap of the transformer through a choke coil. One of the field effect transistors has a drain connected to one end of a primary winding of the transformer and a source connected to a ground line. The other field effect transistor has a drain connected to the other end of the primary winding of the transformer and a source connected to the ground line. Gates of the field effect transistors are connected to the ends of a feedback winding of the transformer, respectively. One end of a secondary winding of the transformer is connected to an end of the discharge lamp through a trigger transformer, and the other end of the secondary winding of the transformer is connected to the other end of the discharge lamp through a resistor.
There are some lighting circuits of different types from the lighting circuit described in the foregoing document. One of the lighting circuits uses a series resonant circuit together with a DC-AC converting circuit. The DC-AC converting circuit generates an AC power having a frequency corresponding to a control signal and the transformer raises a voltage generated in the series resonant circuit. One end of a secondary winding of the transformer and the other end are connected to both ends of the discharge lamp, respectively. Furthermore, one end of the secondary winding is grounded. A control signal is generated corresponding to a voltage to be applied to the discharge lamp (which will be hereinafter referred to as a lamp voltage) and a current to flow to the discharge lamp (which will be hereinafter referred to as a lamp current), and controls a power to be applied to the discharge lamp.
In the lighting circuit, for the lamp voltage and the lamp current, a detecting circuit is not provided on a secondary side of the transformer, but a primary side to which a lower voltage than a voltage on the secondary voltage is applied. In order to control a power to be supplied to the discharge lamp with high precision, however, it is necessary to enhance precision in the detection of the lamp voltage and the lamp current. For this reason, it is preferable that a monitor circuit for monitoring a state of the discharge lamp should not be provided on the primary side of the transformer, but rather should be provided on the secondary side. In the lighting circuit, moreover, it is demanded that accurate monitoring be carried out also when a ground is generated between one end of the discharge lamp and the ground.